Cocklebur will not flower if exposed to 8 hours of dark and 16 hours of light. It will flower if exposed to 16 hours of dark and
8 hours of light. However, if the 16 hours of dark is interrupted with a brief period of light the cocklebur will not flower. What triggers cocklebur to flower
The process explained in the question is called "Photoperiodism" which refers to the plants' reaction to the length of the dark and light period in a day.
When the dark period is interrupted with a certain period of artificial lighting, the plant thinks that the day had a longer period where it's "light" than it is "dark" so it gets tricked into flowering. Same concept can be applied in reverse for the plants who flower during longer periods of dark.
